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Old 01-07-2012, 08:23 AM   #1
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Size of Engine


So much help...thank you so much for all of the feedback on my other posts...really feel that we're learning lots.

Now to size of engine...definitely leaning towards diesel but which one. There are so many different sizes. Someone mentioned that we need to worry about the torque not the size of engine...not sure what that means...could anyone enlighten us?

Is there a minimum size of engine that we should be looking for...we will want to do the mountains on the west coast and will be towing some sort of secondary vehicle.

Also...in someone else's post it was mentioned that you need to figure out how much weight the unit will carry/pull. Is there a formula for this?

Once again....thanks so much for all of the help.

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Old 01-07-2012, 08:56 AM   #2
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Barbra welcome to the forum. This is a hard question to answer without knowing a lot of stats. Usually the MH design engineers has figured all of this out for you. There might be a upgrade on size at a cost to you. Factors like what are you going to pull behind the MH? Horsepower is how the engine is rated, Torque is what does the work and is more important to me. Be careful of a small engine that has been maxed out to show impressive numbers on the brochure. I know this doesn't help, but compare size (litres displacement) is like lungs on a big horse compared to a pony. Both will pull the cart, but which would you want. Our coach weighs in at 41,000 and we have a CAT 400 that does very well. That does not count for the toad weight.
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Old 01-07-2012, 09:10 AM   #3
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I've just read a few more of your other posts, to learn more of what you are trying to achieve. My input to you is this:
-Keep reading this, and other boards as you learn more
-Go look at several kinds of DP at dealers and shows if available
-Read Johnny's RV.NET Class A Stickie on differences between Class A's, it covers lots of solid info
-Determine your budget range, with an added contingency and 'baseline' repairs if going the used route
-Come up with a list of Must Have's and Like to Have's (Tag vs Non Tag, number of slides, side radiator vs rear radiator, kitchen slide vs non kitchen slide, compression brake vs exhaust brake, engine size, pass thru basements vs basements that move out with slide, tank capacity, aqua/hydro hot vs water heater, CCC adequate for your needs, etc.)
-From your review of boards, dealers, shows, etc. Come up with a short list of manufacturers and models that fit within your price budget range. Then go shopping.

Everyone has different wants. For us, we wanted to remain in budget, while still reaching a Tier 1 Quality range of coach. So, this meant we went the used route, and dropped years to obtain quality level we desired while remaining within budget.

As far as engines, buy the rig first, and you will usually find that the engine option in that rig, will do you well. In the RV.Net stickies, and also here and other boards, you will find a general rule of HP to Weight ratio 100 to 1. For example a total weight (Learn understand the buzzwords on all of the different weights like towing, CCC, carrying capacity, etc. This is the total combined weight, including toad, as that is what the engine needs to pull up a grade.) of say 37K Lbs would be supported with a 370HP or above. That is a general rule, and torque makes a difference too.

IMO, if in the rigs that make your short list, you have and option between two different size engines, go with the biggest engine size you can get in that rig. Most medium to big DP's, come with middle block or big block sizes, some offer options to get the big block. (ISM/ISX is bigger then sam ISL or C12-15 are bigger then C7-9). Only a very few years or engines had any real problems to be concerned about. So once you pick your models, see if they are in that range. (ISL400's had a few years of concern, as an example.) And, many of the mid 2002/3 - 2006/7 had problems with aluminum radiators. Not a show stopper, but know that going in as a potential future repair, and price accordingly.)

Best of luck on your search, have fun, and enjoy when you buy!
07 Country Coach Magna Rembrandt 45' ISX600
Roo II was our 04 Country Coach Allure 40'
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Old 01-07-2012, 09:38 AM   #4
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You are suffering from information over load. Proceed very slow by looking at every thing in your perspective coach, sit in all of the seats, operate all of the stuff that will operate, get an explanation on what will not operate, and take it for a long test drive over as many types of roads available. Go home at the end of that day and come back a week later and probe into whatever you missed before. Even drive it again. If the dealer balks at any of this, find a new dealer.

As for the engine size, anything you buy is going to appear slow and heavy and you will not know the difference by driving between the engine sizes. The most responsive engine, and I have owned three of them, is the last Ford V10 I owned. It would out perform any of the three diesels at low altitudes which I have owned. Since the diesel is turbo charged, they will perform better at high altitudes. Fuel mileage is slightly better then the gas. The ride quality of the air suspension which is present on most diesels is also better.

The relationships between torque and horsepower is frequently miss interpreted by numerous people on these posts. If you are not knowledgeable and/or do not understate the laws of physics, then it is better that you proceed by using you own observations(driving, etc) in making your buying decisions. PM me if you would like more information from me on the issues.
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Old 01-07-2012, 09:42 AM   #5
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Hello Barbara. The answers to your questions are pretty subjective. Many people are more than happy with the performance of their smaller engine coaches while others of us will tell you that bigger is definitely better when it comes to engine size and horsepower. As a general rule, gasoline engines are usually identified by their cubic liter displacement (5.7, 6.8, 7.4, etc.) while diesel engines are generally identified by their peak horespower output (275, 300, 350, 400, etc.). Again, generally speaking, gasoline engine performance is consistent with the displacement - the larger the displacement, the more horsepower it will have and the more torque (pulling power) it will have. The pulling power of diesel engines is generally greater as the displacement size and horsepower rating increases. While there are some exceptions to these rules, and there are ways to maximize horsepower and torque from any engine that can affect it's output, in general terms, this is basically the way it works. So, if you plan on doing a lot of mountain and hilly driving while pulling a towed vehicle (toad), more is probably better in terms of horsepower and torque in either a gasoline or diesel engine. Here is my personal experience with diesel engine coaches. I have owned a 275 horsepower, a 300 horsepower (5.7 Liter) and now a 350 horsepower coach. The 350 horsepower 8.3 Liter Cummins diesel engine is the first one that I am happy with as far as performance while towing on mountain roads is concerned. While generally, the lower horsepower coaches got better fuel mileage overall, the fuel mileage on hilly roads is far better with the 350 than the smaller horsepower engines. And, I don't have to limp along in the right lane while even trucks pass me by on the hills as I experienced with the smaller engines. Most newer gas powered coaches have either big block Chevrolet or Ford engines and either do an excellent job. Depending on how big a coach you are looking for, if you stay at 34 feet or shorter, I'm sure any gasoline powered coach would do an adequate job of powering your coach for you. However, in my humble opinion (IMHO), if you are looking at a coach longer than 34 feet, a big block diesel engine is what you should be looking for to power your coach. Again, all this is pretty subjective, but I think using this approach as a guide when shopping for a coach will serve you well.

Regarding the weight you carry/pull. Manufacturers generally will match the engine (and gearing) needed to power their coaches down the road to the weight rating of the coach chassis. What you should be conscious of though is that the Gross Combined Weight Rating (GCWR) - (the weight of your fully loaded coach - usually expressed as Gross Vehicle Weight (GVW) plus the weight of your toad) is not exceeded. So, check on the weight of the vehicle you intend to tow and add that to the fully loaded weight of the coach to make sure you will be within (or close to) the GCWR of the coach you choose. Some will argue that so long as your towed vehicle has its own auxiliary braking system that it's weight should not be a factor. I choose to error on the side of making sure the coach I ride down the road in is within manufacturer's recommended load limits including the weight of my toad.

Good luck to you in your search and keep the Forum posted on what you end up buying. Happy Trails!
Roger & Diana
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Washington, UT
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Old 01-07-2012, 09:53 AM   #6
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The rule of thumb for motor home engine size is 100 lbs per horsepower, e.g. a 30,000 lb coach should have at least 300 HP. That gives adequate performance, but it will still slow down on long steep grades. Some people want more HP to give more car like performance.

Don't worry about HP vs torque - it's mostly something for gear heads to argue over. The two have a fixed relationship and more of one means more of the other as well.
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Old 01-07-2012, 10:32 AM   #7
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Torque rating is actually more important than Horsepower rating. Just look at a 300 hp gas vs 300 hp diesel. The torque ratings are quite a bit different, and so is the pulling power, and ability to climb hill hills. When looking, Torque is always the concern.

If you decide on a MH 35 ft or shorter, gas will be fine. Anything bigger, you will be better served with diesel.
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Old 01-07-2012, 03:19 PM   #8
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Originally Posted by Gary RVRoamer View Post
Don't worry about HP vs torque - it's mostly something for gear heads to argue over. The two have a fixed relationship and more of one means more of the other as well.
But one thing that is important to understand is that a specific horsepower level can be achieved by a big engine loafing along or a small engine working very hard. With diesel engines torque is almost entirely a function of engine displacement (size); the bigger the engine, the more torque. Torque levels vary from a low of ~800 ft-lbs in the small Cummins ISB engine up to ~1800 in the 15 liter ISX engine (or the CAT C-15 equivalent).

A smaller, lower torque engine can be made to produce more HP by designing it to rev faster (diesels are typically very slow-turning engines.) It is a lot cheaper for an RV manufacturer to get 400 HP by installing a "souped up" version of the Cummins ISC engine than it is for them to install the larger ISL engine. Since the average consumer doesn't know much about this stuff, they see "400 HP" and that's all they care about. But the larger ISL engine will provide better low-speed performance (acceleration) and will produce it's 400 HP while loafing along at ~1500-1700 rpm.

Our CAT C-12 is the equivalent of the Cumming ISM engine and when we cruise at ~63-65 mph it is turning something like 1400 rpm. Since it's maximum HP is developed at ~2000 rpm it has a lot of reserve power when it encounters hills or I want to accelerate to pass someone.
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Old 01-07-2012, 03:47 PM   #9
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Hi AloraDanin,
Consider KISSing this question. Read Gary RVRoamer's post and leave it at that. Floor plan sells the coach. Focus on the living part of the coach.
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Old 01-07-2012, 05:26 PM   #10
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Figure out what matters, then go find it. For us it was a tag (comfort while driving, stability in mountains in wind and on icy roads, and carrying capacity), 4 slides (wanted space), aqua hot (engine heats the coach on the road in winter, boiler pre-heats the engine on cold mornings), big engine (torque to move all of the above).

There were lots of acceptable floor plans that met the above requirements, so then it came to price and condition.
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